One of the central problems of modern biology is the understanding of the control of cellular growth in the whole organism. The somatomedins are a group of peptides which have growth promoting actions in a wide variety of in vitro test systems. It is proposed that they have similar effects in vivo and are mediators of growth hormone action. It is the overall goal of this project to examine the state of SM in plasma and the relationship with the purified peptides, the origin of SM, mode of transport in plasma, interactions with target tissue receptor sites, and their in vivo actons. The study will utilize gel chromatography, ion exchange chromatography, isoelectric focusing, and gel electrophoresis. Assay of SM will be carried out utilizing bioassay and a variety of radiolingand assays utilizing 125I-SM. This information will help clarify the role of the somatomedin in growth control.